1. Field of the Invention
This invention relates generally to the communication of data in a multidimensional digital frame structure and, more particularly, to a system and method of selectively scrambling the framed communications as a means of segmenting nodes in a network of connected communication nodes.
2. Description of the Related Art
Non-return-to-zero (NRZ) encoded transmission systems are typically designed so the timing reference clock at the receiver is recovered solely from transitions in the received data signal. Information is reliably communicated over such systems if the transition density of the transmitted data sequence is sufficiently high. If the data sequence transition density is not adequate, the recovered timing reference clock in the receiver will not track the transmitter timing reference accurately enough to receive the data sequence error free. When the transition density is too low, the receiver timing reference becomes “unlocked” with respect to the transmitter timing reference, and communication between the transmitter and receiver is lost or degraded.
In order to ensure that adequate data transition density exists, many NRZ encoded transmission systems rely on scrambling to randomize the data prior to transmission. The SONET format, as described in GR-253-CORE “Synchronous Optical Network Transport Systems: Common Generic Criteria”, Revision 1, December 1997, Bellcore, is an example of such a system. The frame synchronous SONET scrambler described in the above publication works well for Time Division Multiplexed (TDM) based payload mappings that interleave data from multiple sources into a single SONET frame. For payload mappings that accept data from a single source, the SONET frame synchronous scrambler may not be adequate. The SONET format suffers from two liabilities: 1) it is reset to the same value at the beginning of each SONET frame; and, more importantly, 2) the length of the sequence before repeating is only 127 bits.
Publication RFC-1619, “PPP over SONET/SDH”, Issue 1, May 1994, Internet Engineering Task Force, defines a direct mapping of the HDLC (high-level Data link control) encapsulated packet based point-to-point protocol (PPP) into the SONET payload. As is noted in “Self-Synchronous Packet Scrambler”, U.S. Pat. No. 5,835,602, invented by S. Lang, a malicious user may generate packets consisting of the SONET frame synchronous scrambler sequence. If such packets are transported using the mapping defined in RFC-1619, there is a non-trivial probability that the packet sequence will be aligned with the frame-synchronous SONET scrambler resulting in long sequences of ones or zeros that could disrupt the receive clock recovery circuit.
U.S. Pat. No. 5,835,602 describes a method of reducing the probability of the above-described disruption, which involves adding a self-synchronizing scrambler after the HDLC (protocol for X.25 packet switching networks) packet generation, but before the SONET frame generator. This provides protection as long as the malicious user has no knowledge of the state of the self-synchronizing scrambler. However, a user may have knowledge of the scrambler state at the start of transmission. The self-synchronizing scrambler is usually initialized to a pre-defined state (such as all ones). Then, if only HDLC idle flags are passed through the scrambler until the packet transmission is initiated, the self-synchronizing scrambler will be in one of a small number of states when packet transmission begins. Thus, a malicious user could still disrupt transmission with a non-trivial probability of success.
This security problem could be solved if the scrambling algorithms could be constantly modified, so that a non-authorized user could not know the scrambling state. However, the scrambling seeds for these scramblers are not normally programmable. There is no standard practice with respect to the programmability and reloadability of the scrambling seeds for the purpose of providing secure data in a variable rate interleaved multi-frame digital wrapper system using forward error correction (FEC).
It would be advantageous if framed digital communications could be transmitted with greater security from intentional disruption.
It would be advantageous if framed digital communications could be selectively communicated to nodes in a network of nodes.
It would be advantageous if communications could be selectively scrambled with constantly changing scrambling algorithms.
It would be advantageous if the seed masks used to generate scrambling algorithms for transmitted communications could be periodically changed. Likewise it would be advantageous if the communications could be received and descrambled using the same seed masks.